The present invention relates to a current sensor using a Sagnac interferometer including an optical fiber coil set within a magnetic field generated by a current and through which a clockwise and counter-clockwise light are propagated to undergo a Faraday effect to have their polarization planes rotated in mutually opposite directions to produce a phase difference between them, whereby the detection of the phase difference allows the current to be determined.
The measurement of a current through a transmission line used in the art of power transmission and distribution generally employs a transformer comprising an iron core and a coil winding thereon. Since the transformer represents a purely electrical instrument, it is required that the transformer satisfies electrical noise resistance and dielectric strength requirements, and depending on the location where it is installed, a consideration must be paid to the outer profile and dimension.
Sagnac interferometer comprising an optical fiber coil is being investigated and developed for its use in a current sensor which is free from influences of electrical noises and which does not require a dielectric strength to be secured. Sagnac interferometer comprising an optical fiber coil has been used in the art of detecting the rotation of a moving body in an optical fiber gyro application. In addition to detecting the rotation, Sagnac interferometer exhibits a response to a magnetic field generated by a current and such response can be utilized to determine a current. Specifically, when a magnetic field is applied to an optical fiber coil which comprises a transparent material, the Faraday effect causes a rotation of a polarization plane, and an angle of rotation of the polarization plane is proportional to both the strength of the magnetic field and a distance through which a light passes in the magnetic field. A phase difference is produced between dextrorotatory and levorotatory light passing through the optical fiber coil due to a rotation of the polarization plane. The detection of the phase differences allows the magnitude of the current which generated the magnetic field to be determined. A conventional example of a current sensor using a Sagnac interferometer will now be described with reference to FIG. 1.
In FIG. 1, light emitted from a light source 1 passes through an optical directional coupler or a first optical branch unit 2, and further passes through a first polarization filter 3 to a second optical branch unit 4 where it is split into levorotatory and dextrorotatory light to impinge on a current sensing coil 6. The levorotatory light is modulationed in a phase modulator 5, whereupon it passes through a quarter-wave plate 16, impinges on one end of the coil 6, proceeds through the coil 6 as levorotatory light to be emitted therefrom, impinges on a second quarter-wave plate 17, and then successively passes through the second optical branch unit 4 and the first polarization filter 3 to impinge on the first optical branch unit 2 where it is branched into a light receiver 7 to be received thereby. On the other hand, dextrorotatory light from the second optical branch unit 4 passes through the second quarter-wave plate 17 to impinge on the current sensing coil 6, proceeds through the coil 6 clockwise to be emitted therefrom, impinges on the first quarter-wave plate 16 where its optical phase is modulated in the phase modulator 5. The phase modulated dextrorotatory light successively passes through the second optical branch unit 4 and the first polarization filter 3 to impinge on the first optical branch unit 2 where it is branched into the light receiver 7 to be received thereby. It is to be understood that each of the first quarter-wave plate 16 and the second quarter-wave plate 17 converts a linearly polarized light which is incident from the polarization filter 3 into a circularly polarized light which is emitted, and also converts a circularly polarized incident light into a linearly polarized emitted light. It will be noted that a modulation input is input to the phase modulator 5 from an oscillation circuit 9 in order to perform an optical phase modulation of dextrorotatory and levorotatory light.
When an electric wire 10 is brought close to an end of the current sensing coil 6 which is subject to a magnetic field such that the diametrical direction of the coil is on an extension of the wire 10, a phase difference is produced between the dextrorotatory light and the levorotatory light after passing through the coil 6, and the dextrorotatory light and the levorotatory light emitted from the coil are subject to a synthesizing interference in the second optical branch unit 4, with consequence that the light receiver 7 receives phase modulated light having an optical strength which varies in accordance with the phase difference. A change in the strength of the interfered light has a frequency which coincides with the frequency of the modulation signal from the oscillation circuit 9, and a phase which corresponds to the phase difference between the levorotatory light and the dextrorotatory light. Upon reaching the light receiver 7, the phase modulated light is converted into an electrical signal having an amplitude which varies in accordance with the optical strength. The electrical signal which is obtained by the photoelectric conversion is input to a synchronous detector 8. The modulation signal which is supplied to the phase detector 5 is input to the synchronous detector 8 from the oscillation circuit 9 as a reference signal, thus performing a synchronous detection of the output from the light receiver 7 which is input thereto. The synchronous detection output corresponds to the phase difference which is in turn proportional to the magnetic field applied to the current sensing coil 6. (For details of the phase modulation, see Japanese Laid-Open Patent Applications No. 99/351883 and 01/21363.)
As mentioned above, a current sensor using a Sagnac interferometer determines the magnitude of a current which generated a magnetic field applied to a current sensing coil, by causing circularly polarized lights to impinge on opposite ends of the coils and propagate therethrough as a levorotatory light and a dextrorotatory light and causing the both lights having a phase difference therebetween to interfere with each other so that the resulting interfered light has a varying optical strength which can be used to determine the magnitude of the current.
In order for Sagnac interferometer to operate as a current sensor, it is necessary that circularly polarized lights be incident on the current sensing coil 6 as mentioned above. To satisfy this requirement, in the conventional example shown in FIG. 1, optical fibers or fiber portions shown in thick lines are constructed by polarization maintaining optical fibers. Specifically, except for the optical fiber which forms the current sensing coil 6, an optical fiber extending from the light source 1 to the first optical branch unit 2 and having a length on the order of one meter, an optical fiber extending from the first branch unit 2 to the first polarization filter 3 and having a length on the order of one meter, an optical fiber extending from the first polarization filter 3 to the second optical branch unit 4 and having a length on the order of one meter, and an optical fiber extending from the second optical branch unit 4 to the phase modulator 5 and having a length on the order of one meter are each formed by a polarization maintaining optical fiber. Assuming a total length of the optical fiber which forms the current sensing coil 6 to be ten meters, the length of each of optical fibers extending from the second optical branch unit 4 to the first quarter-wave plate 16 and extending from the second optical branch unit 4 to the second quarter-wave plate 17 has a length which is chosen to be about fifty meters, each optical fiber of such length being formed by a polarization maintaining optical fiber. In the conventional example shown, the first optical branch unit 2, the first polarization filter 3, and the second optical branch unit 4 are also formed by polarization maintaining optical fibers, as shown.
It is to be understood that a considerable length of time is required for the alignment of the proper axis of light from the light source 1 with the proper axis of a polarization maintaining optical fiber which connects to the first optical branch unit 2, and accordingly, this current sensor is correspondingly expensive. In addition, a polarization maintaining optical fiber is much more expensive than a singlemode optical fiber which does not maintain polarization. For this reason, the conventional current sensor of Sagnac interferometer type shown in FIG. 1 is expensive.
FIG. 2 shows a current sensor using a Sagnac interferometer including a coil of length adjusting optical fiber added to the conventional example shown in FIG. 1.
In the Sagnac interferometer, a sensing coil comprises a winding of a single mode optical fiber. When it is used as a current sensing coil 6, a coil design having a total length on the order of ten meters will be sufficient for the purpose of detecting the current. In order to achieve a good sensitivity in detecting an electrical signal which is obtained as a result of a photoelectric conversion by the light receiver 7, it is common in Sagnac interferometer that the phase modulator 5 be inserted in one end of the sensing coil 6 to provide an optical phase modulation in an a.c. sense with respect to the levorotatory light and the dextrorotatory light as mentioned previously. When the sensing coil 6 has a length of optical fiber which is on the order of ten meters, there cannot be obtained a sufficient difference in the propagation time between the levo- and dextro-rotatory lights, making it difficult to achieve a sufficient modulation amplitude for the resulting interfered light. To overcome this problem, a length adjusting optical fiber coil 60 is connected in series to one end of the sensing coil 6 so that a length of optical fiber on the order of one hundred meters be established from one branch end of the second optical branch unit 4 to the other branch end thereof including the sensing coil 6 and the length adjusting optical fiber coil 60. Assuming a length of optical fiber for the sensing coil 6 equal to ten meters, the length adjusting optical fiber coil 60 is chosen to have a length on the order of ninety meters. Since an error of a modulation frequency relative to an optimum drive frequency of the phase modulator 5 is proportional to a spike signal width, which is responsible for a bias variation, a product of a length of sensing coil L inclusive of the length adjusting optical fiber coil 60 and the modulation frequency f is generally chosen so as to satisfy the following equation:
f L=c/2n
where c represents a light velocity and n a refractive index of the optical fiber.
Substituting c=3xc3x97108 m/sec and n=1.45, it follows:
f L=100 m MHz
In the current sensor of Sagnac interferometer type shown in FIG. 2, except for the current sensing coil 6 which is to be formed by a single mode optical fiber, optical elements as well as optical fibers which connect between optical elements are formed by polarization maintaining optical fibers indicated by thick lines, and the length adjusting optical fiber coil 60 which is as long as ninety meters is also formed by a polarization maintaining optical fiber. This explains for an expensive price of the entire current sensor of Sagnac interferometer type.
It is an object of the present invention to provide a current sensor using a Sagnac interferometer which overcomes described problems caused by the use of polarization maintaining optical fibers.
The present invention is premised on the current sensor of Sagnac interferometer type in which an emitted light is passed through an optical directional coupler to impinge on a first polarization filter, which emits a linearly polarized light having a given plane of polarization which is then split into two beams in a second optical branch unit, one beam passing through an optical phase modulator and a first quarter-wave plate while the other beam passing through a second quarter-wave plate, whereby the both beams impinge upon the opposite ends of a current sensing coil as a levorotatory light and a dextrorotatory light. According to a first aspect of the present invention, a first depolarizer is inserted between the optical directional coupler and the first polarization filter.
Preferably a second depolarizer and a second polarization filter are inserted between the optical phase modulator and the first quarter-wave plate, and a third depolarizer and a third polarization filter are inserted between the other branch end of the second optical branch unit and the second quarter-wave plate.
According to a second aspect of the present invention, a second depolarizer and a second polarization filter are inserted between the optical phase modulator and the first quarter-wave plate, and a third depolarizer and a third polarization filter are inserted between the other branch end of the second optical branch unit and the second quarter-wave plate. A light source having no polarizing characteristic is used.
Also preferably a first length adjusting optical fiber coil is connected in series between one branch of the second optical branch unit and the first quarter-wave plate, and a second length adjusting optical fiber coil is connected in series between the other branch end of the second optical branch unit and the second quarter-wave plate, the both length adjusting optical fiber coils being wound in opposite directions to each other and having their center axes aligned on a common rectilinear line.
More preferably, the first and the second length adjusting optical fiber coil and the current sensing coil have their center axes which are substantially aligned on a common rectilinear line and phase changes of light occurring by the Sagnac effect to which the three coils are subject canceling each other.
Preferably, a separation is made between the optical directional coupler and the second optical branch unit, which are then connected together through a first optical connector/an extension optical fiber/a second optical connector 22. Alternatively, a separation may be made between the second branch unit and each of the first and the second quarter-wave plate, which can be preferably connected together by a first optical connector/two extension optical fibers/a second optical connector 22.